When you don't make a tube with an rp of 60-70k drive a 100k load, the linearity (not surprisingly) is 10-20 times better than that.

That's the point of using a mu follower.

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Interestingly, in a test where the tubes weren't asked to drive a heavy- for them- load, the no-name ultra short plate 12AX7 (innards look like 12AT7, Eastern European source) tested as well or better than the much-vaunted smooth plate Telefunkens.

Can you provide figures and describe your test circuit?

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I'm surprised that the 6CG7 doesn't do better than that.

Morgan Jones in his tests with a CCS load got figures only 3dB better than this for the intrinsic distortion in a 6SN7. Most tubes do worse.

The good thing about a 6SN7 is that it does as well driving a 10K load but the 12AX7 obviously does not.

OK, that ties up well. Since distortion is directly proportional to output level your 8.5V rms is -7.5dB relative to my 20V rms. I got 2H at -60 which means at your level I would get -67.5dB cf your -70db. And as I said before, the mu follower seems to be no more than 3dB worse than a CCS.

Did you look at the other post I referenced? That gave results at close to 20VRMS out.

Yes, you got -65dB 2H cf my -60dB so that's a bigger 5dB difference - more than I would have expected.

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That particular spectrum was from a Telefunken. As I said in the thread, the tiny plate Eastern European versions gave me almost the same results (within a dB or so).

Which does not tally with my tests where the small (less tall) plates gave distinctly worse results. That said I only tested two tubes of each type all current production. I also tested a couple of EH 12AX7 WA and one gave the amazing result of -68dB 2H at 20V rms - but this was at the expense of higher 3H. I am not quite sure what I was seeing there but I have noticed some other tube examples that have a better/worse 2H component accompanied by a similarly worse/better 3H component.

You mentioned the effect of mu follower load on distortion. This is quite true but also seems to exhibit this trade off between 2H and 3H.

For example, I tested some 6CG7 at 20V rms into loads varying from 100K down to 10K. At the extremes the 2H was identical but 3H increased by about 10dB at 10K load. There was a pronounced 2H minima of several dB at 15K load but again at the expense of 3H (although THD was also at a minimum). Generally from 100K load downwards to 15K, 2H decreased at the expensive of 3H (though THD decreased too) thereafter both increased. I also measured higher order harmonics which were not present at the 100K load but began to appear with heavier loads. I repeated the tests at 10V rms and there were no higher harmonics than the 3rd detectable at any load.